Structural Analysis of Injection-Molded Polyoxymethylene Treated Below a Melting Point Using Field-Emission Scanning Electron Microscopy and Infrared Spectroscopy

Abstract

The heat treatment of an injection-molded polyoxymethylene slightly below the melting point and subsequent isothermal treatment at 130 °C were performed. The polyoxymethylene structure was examined using field-emission scanning electron microscopy (FE-SEM) and polarization infrared (IR) reflection measurements. After the heat treatment, a significant change in the surface morphology was observed, and the reflection spectrum derived from the polariton in the injection direction also changed dramatically. Since the reflection spectrum in the injection direction contains the reflection component of the perpendicular direction and vice versa, the polarization spectra of both directions can be calculated consistently. The mixing ratio of each crossed component and the pure relative permittivity (RP) both parallel and perpendicular to the main-chain direction were determined using the oscillator model. The heat treatment reduced the ratio of the perpendicular component and increased the order structure until just before melting. The structural changes characterized by the two techniques, along with Raman spectroscopy and differential scanning calorimetry (DSC), are discussed.


Azimuth Angle Dependence of Polarized Infrared Spectra of Injection-Molded Polyoxymethylene

“…12,23 As described in our previous report, the mixing ratios of each cross-component and the pure RPs in the main-chain and orthogonal directions were determined by performing least square fitting to the reflection spectra at φ = 0°and φ = 90°. 25 The results of the simultaneous calculation of the reflection spectra at φ = 0°and φ = 90°are shown by the dotted lines in Figure S1 (Supplemental Material), which proves that using a two-phase model can better explain the data on the mixing ratio of each cross-components than using the EMT. For our sample, the mixing ratio was 17.8% for each orientation direction (this was slightly lower than for the previously reported sample, which was possibly due to the different sampling positions).…”

“…This angle should be smaller, but we set this value for comparison with the spectral shapes from our previous microscopic specular reflectance measurements. [22][23][24][25] The polarizer was placed in front of the detector after the beam was reflected off the sample surface, and s-and p-polarized measurements were conducted. We performed azimuth-angle-dependent reflectivity measurements by rotating the sample.…”

“…Our previous report showed that traces of this Reststrahlen band are also observed in the reflection spectra of the direction perpendicular to the injection. 25 Moreover, the in-plane distribution of the orientation angles of the main chain, characterized by Raman measurements, is bimodal, suggesting that the main chains are partially distributed along the transverse direction. Finally, the RPs in the main-chain direction and the direction perpendicular to the main chain were calculated in our previous report, using a two-phase model that simply adds the reflection of each other.…”

“…Finally, the RPs in the main-chain direction and the direction perpendicular to the main chain were calculated in our previous report, using a two-phase model that simply adds the reflection of each other. 25 Nonetheless, the IR response obtained by mixing the main chains and those perpendicular to the main chains was hard to explain by the effective medium theory (EMT).…”

“…12 Furthermore, we have shown that phenomena where the frequency dispersion increases can occur with mechanical processes such as stretching or extrusion, as well as during heat treatment of POM. [23][24][25] These phenomena, coupled with the crystalline nature of polymers, indicate that the above-mentioned occurrences are not necessarily unique. Therefore, it is crucial to clarify the physical characteristics of the spectra, such as the azimuthal dependence of modes with large frequency dispersion, before performing a detailed chemical structural analysis.…”